1. Field of the Invention
The present invention relates to a filter apparatus including a slave gm-C filter formed by operational transconductance amplifiers (OTAs: gm) and capacitors (C) and a master circuit for automatically tuning the frequency characteristics such as a cut-off frequency or a center frequency of the slave gm-C filter.
2. Description of the Related Art
Wide-dynamic-range gm-C filters formed by metal oxide semiconductor (MOS) OTAs and capacitors have been developed since 1984. For these filters, an automatic tuning is required to maintain precise frequency characteristics of the gm-C filters in spite of manufacturing process variations, temperature drift and the like.
A first prior art gm-C filter apparatus formed on a large scale integrated circuit (LSI) chip is constructed by a slave gm-C filter and a master circuit formed by a phase-locked loop circuit for generating a control voltage for automatically tuning the frequency characteristics of the slave gm-C filter in accordance with a reference frequency signal (see: F. Krummenacher et al., “A 4-MHz CMOS Continuous-Time Filter with On-Chip Automatic Tuning”, IEEE J. Solid-State Circuits, Vol. 23, No. 3, pp. 750-758, June 1988). This will be explained later in detail.
In the above-described first prior art gm-C filter apparatus, however, since the operation mechanism of the voltage-controlled oscillator (VCO) of the master circuit is much complex, this filter apparatus including the VCO is also complex.
Additionally, in the above-described first prior art gm-C filter apparatus, parasitic capacitances with realized circuits cannot be ignored, so that it is impossible to maintain a precise relationship between the oscillation frequency of the VCO and the cut-off frequency or center frequency of the slave gm-C filter, particularly, in a low current type filter apparatus where the drive currents of the OTAs are small.
A second prior art gm-C filter apparatus formed on an LSI chip is constructed by a slave filter circuit formed by two different gm-C filters and a master filter circuit formed by two different gm-C filters having the same structures as the gm-C filters of the slave filter circuit (see: JP-10-013188-A). The master filter circuit receives a reference frequency signal and generates control voltages for controlling the OTAs in the gm-C filters of the master filter circuit, so that the amplitudes of the output signals of the gm-C filters of the master filter circuit are made equal to each other. The control voltages are also used for controlling the gm-C filters of the slave filter circuit, thus automatically tuning the frequency characteristics thereof. This also will be explained later in detail.
In the above-described second prior art gm-C filter apparatus, however, since the gm-C filters of the master filter circuit have a similar structure to those of the slave filter circuit, when the slave filter circuit is constituted as a higher-order filter, the master filter circuit is large in circuit-scale, which would increase the manufacturing cost and the power consumption.